Project Summary Understanding the transcriptional regulation of polarized T cell responses and their link to inflammatory conditions and resistance to infection has been a major theme in immunology for the last 20 years and has led to the identification of T-bet, GATA3 and RoR?t as ?master regulators? of the development of Th1, Th2 and Th17 type responses, respectively. As such, the ability of T-bet to promote IFN-? production has dominated our appreciation of how T-bet contributes to T cell responses in the setting of infection. This is reinforced by the increased susceptibility of T-bet-/- mice to various intracellular infections, including the opportunistic parasite Toxoplasma gondii. In CD4+ and CD8+ T cells there is good evidence that T cell activation is accompanied by two waves of T-bet expression (the T-bet two step) and that the initial TCR- mediated induction of T-bet sensitizes T cells for polarizing signals provided by cytokines such as IL-12 that reinforce further T-bet expression and commitment to differentiation. However, our recent published and preliminary studies indicate that in CD8+ T cells T-bet may have an unanticipated role in controlling a myriad of early T cell activation induced events including upregulation of adhesion molecules and chemoattractants and entry of CD8+ T cells into cell cycle. These results have led to the novel hypothesis that the early induction of T-bet optimizes the interactions between recently activated T and DC populations to promote entry into the cell cycle, optimize expansion and the acquisition of effector functions. To test this hypothesis we will combine transcriptional profiling to identify early targets of T-bet with state of the art cellular immunology and imaging approaches to understand the role of T-bet and in hand candidates in the initial events that are essential for the generation of effector responses required for resistance to an intracellular pathogen.